Hm. This seems exactly wrong to me. There are almost zero feats other than Elemental Adept that are designed to work around monster defenses.
Zero?
I consider AC to be a monster defense.
I consider hit points to be a monster defense.
I consider a high init mod to be a monster defense.
There are quite a few feats that give advantage or do bonus damage or a variety of other things to take out monsters faster with. Alert, Charger, Crossbow Expert, Dual Wielder, etc. Probably a third of all feats are offensive, hence, they "break" the normal rules against the defenses of monsters.
Elemental Adept is just a special corner case of these more general case anti-monster defense feats (pro-PC offense = anti-NPC defense).
Give WotC time. They will eventually handle most or all of the special defense corner cases.
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The Dragon in the Starter Set isn't a threat just because of its offensive weaponry, it's a threat because it has so many hit points that the PCs have virtually no chance of defeating it. It has a huge hit point defense.
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